In various comparative studies or research applications, it is oftentimes necessary to apply multiple sets of electrodes to compare the performance of acquisition devices or to obtain an extra set of physiological signals which are to be separately processed. The current practice is to apply the second set of electrodes next to the first set of electrodes on the patient or to cut excess portions of the adhesive pad from an electrode so that the signal acquisition portion of each of the electrodes are as close together as possible. Because an electrode receives electrical signals which represent vectors of the electrical signals of the heart, this type of arrangement may introduce an inherent bias or variability into the acquired data. If the electrodes are offset from each other, the signals that are to be compared will be inherently different and will vary slightly based on the spacing of the electrodes. In certain research projects or where the equivalence of a new acquisition device is being compared to an existing commercial device this difference in the location that the signals are acquired from may require duplicate testing with each device being connected to each electrode location. Alternately, the differences may have to be estimated from a single test and then separately analyzed to determine if the differences in the acquired signals represent a statistically significant difference. Neither approach is particularly appealing and may be affected by a variety of factors which must either be accounted for or ignored.
Another difficulty with current practice of electrode placement is that motion artifacts have long been a problem during the measurement of biopotentials, particularly in long-term electrocardiogram (ECG) monitoring of coronary care patients and in exercise (stress) ECG's. Motion artifacts can be defined as motion induced fluctuation of the electrical potential across the skin of the patient. Motion artifacts manifest themselves as electrical interference which is often superimposed on the desired physiological signal and minimizes the usefulness of the physiological signal for diagnostic and clinical purposes. Motion artifacts are generally caused by the movement of the patient relative to the electrode applied to the patient's skin, thereby disturbing the skin potential and creating extraneous readouts on the ECG monitor which either mask or cause a shift in the baseline of the desired physiological signal. In clinical and comparative studies, the existence of motion artifacts may introduce yet another variable which must be ignored or accounted for in the study results. Additionally, if one acquired signal is closer to the source of the motion artifact than the other acquired signal, the comparison of the two signals may required increased filtering of one signal or other special treatment which may affect the comparative analysis of the study results.
It is well known that light abrasion of the skin reduces the electrical potential and minimizes the impedance of the skin of the patient, thereby reducing motion artifacts and improving signal or trace quality of the physiological signal. Although there are many commercially available surface mounted electrodes for cardiac monitoring described in the literature, reliable signals or trace results from these electrodes in highly dependent on adequate skin preparation prior to application of the electrodes. Proper skin preparation is time consuming because typical stress electrocardiograms usually require between three and 12 electrodes and typically use about 10 electrodes and research and comparative studies may use double that amount. Skin preparation is normally necessary to remove the epidermal layer of the skin of the patient and is carried out in a variety of ways. The most common method of preparing the skin is to rub the patient's skin with a gritty material contained in a carrier or to rub the patient's skin with a rough surfaced material to which an antiseptic such as alcohol or other solvent is applied. Other approaches include chemical preparations or a variety of mechanical abrasion. After briskly rubbing the skin, the skin is dried and again rubbed with a dry cloth. If, after the electrodes are applied, a proper signal or trace is not obtained from one or more of the electrodes, the malfunctioning electrodes must be identified, removed and the skin must be prepared again. The electrodes are then reapplied to the skin of the patient, and this procedure is repeated until an adequate and accurate signal is received from each electrode. The effectiveness of the skin preparation is highly dependent on the technique used as well as the level of skill of the person preparing the skin. Predictably, the effectiveness of the skin preparation in this uncontrolled manner is highly variable between electrode locations as well as between patients and represents yet another variable which may affect the results of the research or comparative study.
U.S. Pat. Nos. 4,274,419 and 4,311,152 are owned by the assignee of the present invention and disclose a single surface mounted medical electrode suitable for recording physiological measurements in which the electrode is first applied to the patient, and then the skin of the patient is prepared. Such an approach markedly reduces the time consumed in the application of electrodes for recording physiological events. Also, more reliable, accurate and uniform signals are obtained since the amount and type of skin preparation for each electrode is generally uniform.